A simple physics problem, please help!

AndrewD

AndrewD

AndrewD

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Hi,

I have an entrance exam on friday and seem to have most of the bases covered apart from physics which is definately my weak point, fortunately it's the lowest scoring section of my exam, still I'm stuck with a question which regards Torque, or I think it's a torque problem. Here's the link to a *.jpeg of the problem;

1pyluh.jpg


If anyone on here would be kind to show me how to generally solve these problems as it seems to be a popular topic I would be grateful!!.

By the way also, the answers for the questions are C & A, respectively, however there is no explaination in my practice paper on how to tackle this problem.

Thanks in advance.
 
for question one simply ignore that the arm is a lever with pivot points. it says (in a pretty poor way) that the arm is held stationary, with a 250N force pulling upwards from the muscle. it also says that the arm weighs 25N which means you have the force of mavity pulling downwards with a force of 25N. so, to keep the arm stationary there needs to be a (250-25)N force from somewhere pushing it down. the elbow is the only place it can come from, so you've got a force of 225N pushing down from the elbow.

for question two i will need to know what value you are using for mavity (i could probably work it out now that you've given the answer, but it would be a pain in the bum)
 
Okay, the first one is easy peasy :) It doesn't involve torque.
The arm is exerting a 25N downward pull. The bicep is exerting a 250N upward pull.
250-25 = 225N (the upward force must always balance the downward force or things move).

For the second question, it does involve torque. First, convert the 5kg to 50N. Multiply it by the distance (0.35m) to get a 17.5 turning force on the joint. Divide by 0.05m to get the force applied to the biceps (350N). Add the 250N that the bicep is already applying and you get 590N.

Hope that makes sense.
Edit: oh, beaten for the first question. By miles.

For converting kg to N, unless you are actually studying physics it will probably be treated as 1kg = 10N. If that doesn't work, use 1kg = 9.81N.
 
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As far as I can make out, since the arm is in static equilibrium, the two opposing forces are at 5cm from the elbow (bicep pulling up, say B) and 35cm from the elbow (mass pushing down, say M). We can dispense with forces at angles since we're assuming strictly upward and downward forces.

This makes:

B(5cm) - M(35cm) = 0
B = 7M

Where M = 5kg = 49N (Newtons)

B = 343N

Add that to the original 250N to give 593N.

...I think.

Edit: Maybe I went the long way round!
 
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mistercrabb said:
For converting kg to N, unless you are actually studying physics it will probably be treated as 1kg = 10N. If that doesn't work, use 1kg = 9.81N.
Click to expand...

yep, i know that. i did physics and mechanics(maths) in A level :). however, depending on what exam board you do physics with, g could be 10, 9.8, 9.81, or even accurate to 10dp (which i cant be bothered to look up).
it was really confusing having to use 9.8 in mechanics and 9.81 in physics :(
 
Reaper 392 said:
yep, i know that. i did physics and mechanics(maths) in A level :). however, depending on what exam board you do physics with, g could be 10, 9.8, 9.81, or even accurate to 10dp (which i cant be bothered to look up).
it was really confusing having to use 9.8 in mechanics and 9.81 in physics :(
Click to expand...

Unless the paper specifically says what value to use for g, just stick an assumption at the start of your answer saying you've used a value of 10 for g.
 
Vonhelmet said:
Unless the paper specifically says what value to use for g, just stick an assumption at the start of your answer saying you've used a value of 10 for g.
Click to expand...

By that logic you could always set g to 0, i.e. in space, which makes most fo these calculations much easier;)
We always used 9.81 or 9.807.
 
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